The ability of silicon-based microelectronic integrated circuits to achieve higher data rates while simultaneously maintaining low power consumption requirements may be limited by conventional electronic interconnection technologies. To address this and other limitations associated with conventional electronic interconnection technologies, attempts have been made develop integrated optical interconnects because they offer the possibility of lower power-consumption requirements, lower data skew and higher bandwidths.
In particular, attempts have been made to provide integrated optical interconnects using monocrystalline silicon as a waveguide material. For example, in an article by R. A. Soref et al. entitled All-Silicon Active and Passive Guided-Wave Components for .lambda.=1.3 and 1.6 .mu.m, IEEE Journal of Quantum Elec., Vol. QE-22, No. 6, pp. 873-879, June (1986), end-coupled planar and channel optical waveguides were demonstrated using monocrystalline silicon layers on heavily doped silicon substrates. In addition, articles by B. L. Weiss et al. entitled Optical Waveguides in SIMOX Structures, IEEE Photonics Tech. Lett., Vol. 3, No. 1, pp. 19-21, January (1991); A. Rickman et al. entitled Low-Loss Planar Optical Waveguides Fabricated in SIMOX Material, IEEE Photonic Tech. Lett., Vol. 4, No. 6, pp. 633-635, June (1992); and U.S. Pat. No. 4,789,642 to Lorenzo et al. entitled Method for Fabricating Low Loss Crystalline Silicon Waveguides By Dielectric Implantation, disclose planar optical waveguides formed on silicon-on-insulator (SOI) substrates using separation by implantation of oxygen (SIMOX) fabrication techniques. However, SOI waveguide technologies typically limit the formation of all optical interconnects to the same level as the integrated electronic devices and therefore limit the "real estate" available for the electronic devices. SOI-based technologies also generally use relatively complicated regrowth techniques for the integration of optical emitters and detectors which are used with optical interconnects.
Other waveguide technologies include: ARROW (anti-resonant reflector optical waveguide), as described in an article by Y. Kokubun et al. entitled Low-Loss Antiresonant Reflecting Optical Waveguide on Si Substrate in Visible-Wavelength Region, Electron. Lett. Vol 22, pp. 892-893 (1986); and glass waveguides on silicon substrates, as described in an article by R. Adar et al. entitled Measurement of Very Low-Loss Silica on Silicon Waveguides With a Ring Resonator, AppI. Phys. Lett., Vol. 58, No. 5, pp. 444-445, February (1991). However, these waveguides technologies are typically complex and may require the use of waveguides having large cross-sectional dimensions to compensate for small index of refraction ratios, as will be understood by those skilled in the art.
Waveguide technologies based on polycrystalline silicon have also been briefly considered but rejected even though (i) multiple layers of polycrystalline silicon can be easily fabricated without requiring complicated regrowth techniques to achieve high integration levels, and (ii) small waveguide dimensions can be obtained because of the large index of refraction ratio (3.5/1.5 for polycrystalline silicon on silicon dioxide SiO.sub.2). This is because polycrystalline silicon typically absorbs heavily at wavelengths used for optical communication (i.e., .lambda.=1.3-1.55 .mu.m). These high absorption levels in polycrystalline silicon have been attributed to loss parasitics such as optical scattering which can be caused by surface imperfections and grain boundaries which are inherent in polycrystalline silicon. For example, the published absorption measurements in polycrystalline silicon are near 1000 dB/cm at optical communication wavelengths, as described in articles by W. B. Jackson et al. entitled Density of Gap States of Silicon Grain Boundaries Determined by Optical Absorption, AppI. Phys. Lett., Vol. 43, No. 2, pp. 195-197, July (1983); and R. E. Jones et al. entitled Electrical, Thermoelectric, and Optical Properties of Strongly Degenerate Polycrystalline Silicon Films, J. AppI. Phys., Vol. 56, No. 6, pp. 1701-1706, September (1984).
Thus, notwithstanding the above-described attempts to develop optical waveguides for optoelectronic integrated circuit applications, there continues to be a need for improved methods of forming optical waveguides which provide for relatively low-loss interconnects and are compatible with conventional processing techniques.